1. Field of the Invention
The present invention relates to a method of burning in a semiconductor device used in a microwave region, particularly in a region at a frequency of not less than 1 GHz.
2. Description of the Related Art
While compound semiconductors such as GaAs are used for transistors used in microwave regions (several hundreds of MHz to 100 GHz), particularly in high-frequency region at a frequency of not less than 1 GHz, such compound semiconductors have various energy levels on the surface and inside. Some of these levels experience chronic changes such as diminishing concentrations due to electric stress given to the transistor, and therefore a burn-in process is carried out wherein electric stress is given to the transistor to cause changes beforehand in order to ensure reliability of the transistor.
FIG. 14 is a schematic diagram of a burn-in apparatus of the prior art. In the drawing, reference numeral 1 denotes a transistor, 2 denotes a package, and 3 denotes a matching circuit at the operating frequency. The semiconductor device being burned in is an internally matched FET having a matching circuit incorporated in the package.
Burn-in is carried out, with a predetermined drain voltage being applied to a transistor 1, by amplifying a frequency signal (for example 20 GHz) which is output from a signal source 5 by means of an amplifier 7. The signal is supplied to a gate of the transistor 1 while maintaining this state for a predetermined period of time and monitoring the power of the input and output microwave signal by means of a power monitor 6.
During such a burn-in operation, unless frequency matching is accurately done, a reflected wave is generated due to the parasitic inductance and capacitance of jigs and wirings, thus making it impossible to feed high-frequency signals to the transistor, and consequently it becomes difficult to carry out such matching in high-frequency region not lower than 1 GHz. Also it becomes necessary to fabricate wiring of the matching circuit with a high dimensional accuracy in order to match the impedence. This results in higher cost of the matching circuit itself and higher total cost including peripheral devices such as the amplifier.
Meanwhile the present inventors have found that, while burn-in is carried out with a high-frequency electric stress applied, the effect of burn-in may also be achieved by using a frequency lower than the operating frequency, that the impurities do not respond to frequencies lower than a certain value and are mobilized thus making it impossible to achieve the same burn-in effect as that carried out at the operating frequency.
That is, because impurities have different response frequencies (reciprocal of response speed), the impurities cannot follow the changes in the electric field and move accordingly in the case of burn-in with an alternating electric field of a frequency higher than the response frequency of the impurity level. When the burn-in frequency is set lower than the response frequency, however, the impurities move in a behavior different from that observed during burn-in at the operating frequency where the impurities do not move, thus the desired burn-in effect cannot be achieved.
According to the inventor's knowledge, the frequency at which the impurities at the impurity level begin to move is 100 MHz or lower for semiconductors such as GaAs, and the physical behavior of impurities remains the same whether the frequency is 1 GHz or 100 GHz, if the frequency is higher than 100 MHz.
At a frequency higher than 100 MHz, thermal effects can be prevented because this frequency is sufficiently higher than the transient thermal response frequency of semiconductor which is several megahertz.
Based on the finding described above, it is assumed that a burn-in effect similar to that achieved at the actual operating frequency, for example 18 GHz or 40 GHz, can be obtained by burning in at a frequency higher than 100 MHz.
Japanese Patent Kokai Publication No. 60-33066 discloses a method of burning in by applying a low-frequency signal (10 MHz or lower) to a gate electrode of a high-frequency transistor. However, because impurities at the impurity level begin to move at a low frequency, such as 10 MHz or lower, it will not be possible to achieve the same burn-in effect as that obtained with the operating frequency (100 MHz to 20 GHz).